A source tells E! News the party started around 6 p.m. and lasted until midnight. "People were very casual, dressed casual, and came with presents," a source added. "About 50 people came, and some brought their kids."

Justin popped the question to his Wanderlust costar back in August. She (of course) said yes, tearing up when she talked about the special night on Chelsea Lately two months later.

We couldn't be happier for the future Mr. and Mrs. Theroux! Now, anybody know how to score a wedding invite?